Position ensuring system for oblique machining in five-axis machine tool

ABSTRACT

A position ensuring system includes an A-axis calibration system which measures a displacement angle, which is an error between a target value and a measured value of the pivot angle of the spindle head about the A-axis, and corrects the pivot angle about the A-axis in such a manner that the displacement angle as measured with the corrected pivot angle as a target value fall within a tolerable range. A corrected data storage device stores the corrected pivot angle about the A-axis. An A-axis control system reads out the corrected pivot angle about the A-axis, the corrected pivot angle about the A-axis to pivot the spindle head when executing oblique machining of the inclined hole.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/937,742 filed Nov. 9, 2007. U.S. application Ser. No. 11/937,742claims priority to Japanese Patent Application No. 2006-305648 filedNov. 10, 2006. The entire contents of all of the applications mentionedabove are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a position ensuring system for afive-axis machine tool, and more particularly to a system which, whenmachining an inclined hole in a workpiece in a five-axis machine tool,such as a bridge-type machine tool having a pivotable spindle head, cancompensate for an error in an A-axis, the pivot axis of a spindle head,and an error in a C-axis for indexing of a table.

BACKGROUND ART

A bridge-type machine tool is conventionally known as a typicalfive-axis machine tool. A bridge-type machine tool includes a spindlehead provided on a cross rail and has, in addition to X-axis, Y-axis andZ-axis, an A-axis for pivoting of the spindle head and a C-axis forindexing of a table. An example of such a bridge-type machine tool isdisclosed in Japanese Patent Laid-Open Publication No. 2004-34168.Five-axis machine tools, including the bridge-type machine tool, havebeen advantageously used for machining on a free-form surface, astypified by machining of a propeller.

In value-added machining of a mold, for example, shaping machining on afree-form surface has been the highest priority, and high-speed rotationof a spindle and high-speed following in axial movement have beenrequired. To meet the requirements, higher-speed and higher-precisionshaping machining with a five-axis machine tool has become realized.

These days, the environment surrounding manufacturing industry ischanging greatly, and there is an increasingly stricter demand forshortening manufacturing time for machined products. There is a also astronger demand by users for a five-axis machine tools that can betterperform process-intensive combined machining. Such demands have led to asignificant improvement in high-speed, high-precision machining, asdescribed above. On the other hand, old-fashioned machining operationsare still practiced in parallel, and the imbalance is becoming aproblem.

For example, in machining of a mold for molding a large-sized resinproduct, such as an instrumental panel or a bumper of an automobile,besides advanced shaping machining, there are many machining operationsfor which advanced shaping machining is not necessarily required, suchas machining of a hole for insertion of an extrusion pin, machining of acooling cavity, undercut-shaping machining, etc.

Even today when high-speed machining is well-established, machiningoperations which are in no way high-speed and high-precision machining,such as machining of an extrusion pin hole, are currently practiced in alabor-intensive manner by skilled workers. This is because a number ofextrusion pin holes are provided in a mold, and that the respective pinholes differ in the inclination and the direction.

To machine an extrusion pin hole with a five-axis machine tool, it isnecessary to pivot the spindle to meet the inclination of the hole andto rotate the table to meet the direction of the hole. However, themachining inevitably involves an error in the tilt angle of the spindlehead due to the weight of the spindle head, or a mechanical error in therotation angle of the table. Conventional five-axis machine tools arethus not suited for machining of extrusion pin holes. Accordingly, alabor-intensive oblique drilling operation by a skilled worker ispracticed separately from shaping machining with a five-axis machinetool.

In practice, for oblique drilling of extrusion pin holes, a skilledworker manually performs setup and drilling for each of the holes. Mostof the operation time is spend on the setup work though only a shorttime is needed for actual drilling operation. Thus, the high-speed andhigh-precision performance of current machine tools is not fullyutilized at present.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a positionensuring system for oblique machining in a five-axis machine tool, whichsolves the above problems in the prior art and which, based on theinclination and the direction of an inclined hole to be machined,carries out measurement of an error in the pivot angle of a spindle headand an error in the rotation angle of a table before initiatingmachining of the hole, and can automatically correct the pivot angle ofthe spindle head and the rotation angle of the table upon machining ofthe hole, thereby precisely ensuring the position of the spindle headand the position of the table in the machining.

In order to achieve the object, the present invention provides aposition ensuring system for correcting an error, caused by theinfluence of gravity and a mechanical error, in a five-axis machine toolhaving, in addition to X-axis, Y-axis and Z-axis, an A-axis for pivotingof a spindle head and a C-axis for rotation of a table, thereby ensuringthe position of the spindle head in oblique machining of inclined holesin a workpiece, said system comprising: an A-axis calibration means formeasuring a displacement angle, which is an error between a target valueand a measured value of the pivot angle of the spindle head about theA-axis as the spindle head is pivoted to meet the inclination angle ofan inclined hole to be machined in a workpiece, and correcting the pivotangle about the A-axis in such a manner that the displacement angle asmeasured with the corrected pivot angle fall within a tolerable range,before initiating machining of the workpiece, the measurement ofdisplacement angle being carried out for all the inclined holes to bemachined by execution of a measurement program prepared based on data onthe shapes, including the inclination angles and the directions, of theinclined holes to be machined; a corrected data storage means forstoring the corrected pivot angle about the A-axis for each of theinclined holes to be machined; and an A-axis control means for readingout the corrected pivot angle about the A-axis for each inclined holeupon executing a machining program for the workpiece by means of an NCapparatus, and instructing the corrected pivot angle about the A-axis topivot the spindle head when executing oblique machining of the inclinedhole.

In a preferred embodiment of the present invention, the positionensuring system further comprises: a C-axis calibration means formeasuring a displacement angle, which is an error between a commandvalue and a measured value of the rotation angle of the table about theC-axis as the table is rotated to meet the direction of an inclined holeto be machined in the workpiece, and correcting the rotation angle aboutthe C-axis in such a manner that the displacement angle as measured withthe corrected rotation angle fall within a tolerable range, themeasurement of displacement angle being carried out for all the inclinedholes to be machined by execution of the measurement program; acorrected data storage means for storing the corrected rotation angleabout the C-axis for each of the inclined holes to be machined; and aC-axis control means for reading out the corrected rotation angle aboutthe C-axis for each inclined hole upon executing the machining program,and instructing the corrected rotation angle about the C-axis to rotatethe table when executing oblique machining of the inclined hole.

According to the present invention, the position ensuring system of thepresent invention enables oblique machining which is entirely differentfrom the conventional oblique machining method in which a skilled workermanually performs drilling of inclined holes after manually performingsetup for each hole to meet the inclination angle and the direction ofthe hole. Thus, the present system makes it possible to carry outliterally NC-controlled, high-precision automated machining of all theinclined holes to be machined in a workpiece merely by executing ameasurement program and a machining program which are prepared based onCAD data. Efficient oblique drilling, fully making use of the high speedof a five-axis machine tool, thus becomes possible. A remarkableincrease in the efficiency of machining of a workpiece can be realizedespecially when the workpiece is a large-sized mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a bridge-type machine tool to which aposition ensuring system according to the present invention is applied;

FIG. 2 is a perspective view of a workpiece to be machined by thefive-axis machine tool of FIG. 1;

FIG. 3 is a cross-sectional diagram illustrating an extrusion pin holeto be machined in the workpiece of FIG. 2;

FIG. 4 is a diagram illustrating the inclination angle and the directionof the extrusion pin hole of FIG. 3;

FIG. 5 is a block diagram of a position ensuring system according to anembodiment of the present invention;

FIG. 6 is a flowchart of a program for measuring a displacement angle inthe pivot angle of a spindle head about A-axis;

FIG. 7 is a diagram illustrating a change in the position of the spindlehead during a measurement/calibration process;

FIG. 8 is a diagram illustrating a displacement angle about A-axis;

FIG. 9 is a flowchart of a program for measuring a displacement angle inthe rotation angle of a table about C-axis; and

FIGS. 10A through 10C are diagrams illustrating a change in the positionof the table during a measurement/calibration process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of a position ensuring system for obliquemachining in a five-axis machine tool according to the present inventionwill now be described with reference to the drawings.

FIG. 1 shows a bridge-type machine tool, an exemplary five-axis machinetool to which a position ensuring system for machining of inclined holesaccording to the present invention is applied.

In FIG. 1, reference numeral 2 denotes a pair of columns and referencenumeral 4 denotes a bed. A cross rail 6, bridging the columns 2 andextending horizontally, is mounted to the columns 2. The cross rail 6 isdesigned to be vertically movable. A saddle 8 is horizontally movablymounted to the cross rail 6. A spindle head 10 is pivotably mounted onthe saddle 8 and is driven by a swivel pivot mechanism supported by apivot rolling guide.

A table 12 is provided on the bed 4. The table 12 is a rotary tablecapable of 360-degree successive rotation, and is capable of turning aworkpiece on the table 12 to an arbitrary direction.

Such a bridge-type machine tool has three linear axes, X-axis, Y-axisand Z-axis. The X-axis is a control axis for feeding the table 12backward and forward, the Y-axis is a control axis for feeding thesaddle 8 in the lateral direction, and the Z-axis is a control axis forfeeding the cross rail 6 vertically. In addition to the X-axis, Y-axisand Z-axis, the machine tool has an A-axis as a pivot axis for pivotingthe spindle head 10 through 30 degrees at the maximum to the right andleft in the Y-Z plane, and a C-axis as a rotation axis for rotating thetable 12 through an arbitrary angle in a successive manner.

FIG. 2 shows an example of a workpiece in which inclined holes aremachined by the bridge-type machine tool. In this embodiment theworkpiece is a large-sized mold 20 for molding a large-sized resinmolded product, such as an instrumental panel of an automobile. Such alarge-sized mold 20 necessitates the use of extrusion pins for taking amolded product out of the mold. A number of extrusion pins need to beinserted into the large-sized mold 20 to take a molded product out ofthe mold. Accordingly, after carrying out shaping machining of a curvedcavity for molded product to form the mold 20, a number of pin holes forinsertion of extrusion pins are machined by oblique drilling in the mold20.

FIG. 3 is a cross-sectional diagram of the mold 20, illustrating theshape of a pin hole. An extrusion pin 21 is comprised of an insert 22and a slide rod 23, and the insert 22 projects from the mold 20 toextrude a molded product. A slide rod hole 24 in which the slide rod 23slides and a core pocket 25 in which the insert 22 is housed aremachined in the mold 20. The slide rod hole 24 and the core pocket 25,as a whole, form a pin hole.

Such a pin hole is usually inclined. With respect to the slide rod hole24 shown in FIG. 4, for example, the hole 24 can be identified at leastby data on the coordinates of a reference point set for the hole, theinclination angle θ of the hole 24, the direction φ of the axis of thehole 24, etc.

When placing the mold 20 on the table 12 and machining the slide rodhole 24 with a drill, the table 12 is rotated so as to meet thedirection of the hole 24 and the spindle head 10 is kept in a tiltedposition meeting the inclination angle of the hole 24, and the drill isfed while numerically controlling the X-axis, the Y-axis and the Z-axissimultaneously, thereby machining the slid rod hole 24.

It is theoretically possible to formulate a program for machining allthe inclined holes to be machined in the mold 20 from data on thecoordinates of the reference points of the respective holes and on theinclination angles, directions and shapes of the respective holes, andto automate machining of all the holes in the mold 20 by execution ofthe machining program.

In fact, however, deflection is produced in the spindle head 10, thecolumns 2 and the cross rail 6 when the spindle head 10 is in a tiltedposition, causing a small error in the tilt angle. Furthermore, amechanical error is inevitably produced when the table 12 is rotated.Because of such a tilt angle error and a directional error, it ispractically very difficult to perform precision oblique machining.

Therefore, the bridge-type machine tool of FIG. 1 is provided with aposition ensuring system according to the present invention, whichensures the position of the spindle head and the table in carrying outoblique machining, to correct errors that will be produced in the pivotangle of the spindle head 10 and in the angle of rotation of the table12 due to the influence of gravity and a mechanical error.

FIG. 5 is a block diagram of a position ensuring system according to thepresent invention.

In FIG. 5, reference numeral 30 denotes a CAD/CAM machine, and referencenumeral 40 denotes a CNC apparatus. The CAD/CAM machine 30 and the CNCapparatus 40 are connected by a communication means, such as serialcommunication or LAN.

The CAD/CAM machine 30 comprises a CAD data preparation unit 31 forpreparing CAD data on the designing of a workpiece to be machined, whichis the large-sized mold in this embodiment, a machining programpreparation unit 32 for preparing a machining program for pin holemachining based on data on the positions of the reference points, theinclination angles, the directions, the shapes, etc. of pin holes,contained in the CAD data, and a measurement program preparation unit 33for preparing a measurement program to perform calibration in order toobtain data necessary for correcting the pivot angle of the spindle head10 or the angle of rotation of the table 12 based on the CAD data.

The CNC apparatus 40 is a CNC apparatus capable of synchronous five-axiscontrol of X-axis, Y-axis, Z-axis, A-axis and C-axis, and basicallycomprises an input/output unit 42, an arithmetic and control unit 43, astorage unit 44, an X-axis control unit 45, a Y-axis control unit 46, aZ-axis control unit 47, an A-axis control unit 48, and a C-axis controlunit 49.

The arithmetic and control unit 43, besides execution of the machiningprogram, executes the measurement program in a user-specified manner. Onexecution of these programs, the X-axis control unit 45, the Y-axiscontrol unit 46, the Z-axis control unit 47, the A-axis control unit 48and the C-axis control unit 49 issue commands for the respective axes,and the commands are outputted to an X-axis servo motor 50, a Y-axisservo motor 51, a Z-axis servo motor 52, an A-axis servo motor 53 and aC-axis servo motor 54, respectively. The actual positions of therespective axes are detected by position detectors 55, 56, 57, 58, 59,and the position detection signals are fed back to the CNC apparatus 40.

When performing calibration by execution of the measurement program, atouch probe 60 is mounted to the font end of the spindle head 10. Acalibration gage 62 is provided at a predetermined position on the table12. The calibration gage 62 is a gage for measuring an error in thepivot angle of the spindle head 10 when it is pivoted. A pair ofcalibration gages 64 a, 64 b is also disposed on the table 12 atsymmetrical positions with respect to the axis of rotation of the table.The calibration gage 62 is a spherical gage having high roundness,whereas the calibration gages 64 a, 64 b are cylindrical gages.

The touch probe 60 is provided with a terminal 61, and an on/off signalgenerated upon contact of the terminal 61 with the calibration gage 62,64 a or 64 b is inputted via a programmable logic controller 65 into theCNC apparatus 40.

FIG. 6 is a flowchart showing the flow of a process of a measurementprogram that performs calibration to determine a correction amount ofthe pivot angle of the spindle head 10 about the A-axis when the spindlehead 10 is pivoted.

FIG. 7 is a diagram illustrating a change in the position of the spindlehead 10 upon execution of the measurement program. The center position P(X0, Y0, Z0) of the calibration gage 62 mounted on the table 12 has beenmeasured and thus is known.

Inclined holes A1 to An are to be machined in the mold 20, with theinclination angles θ and the directions φ of the inclined holes A1 to Anbeing represented as θ1 to θn and as φ1 to φn, respectively.

First, a pivot angle of the spindle head 10 about the A-axis is set(step S1). The angle is first set at the inclination angle θ1 of theinclined hole A1. A macro program is then executed which causes thespindle head 10 to pivot until the pivot angle reaches the inclinationangle θ1 of the first inclined hole A1 while keeping the terminal 61 ofthe touch probe 60 in contact with the spherical surface of thecalibration gage 62 (step S2). By the execution of the macro program,the pivot angle of the spindle head 10 about the A-axis theoreticallybecomes θ1 and the spindle head 10 becomes tilted as shown in FIG. 7.

In step S3, a macro program is executed which involves measuring ameasured center point position P′ (see FIG. 8) of the calibration gage62 while keeping the spindle head 10 in the tilted position. In themacro program, the coordinates of four contact points between the touchprobe 61 and the surface of the calibration gage 62 are measured, andthe coordinates of the center position P′ is calculated from themeasured coordinates of the four points.

After thus determining the coordinates of the measured center positionP′, a displacement angle c about the A-axis can be determined from thedifference (error) from the known center position P in the followingmanner (step S4).

In FIG. 8, assuming the distance from the pivot center of the spindlehead 10 to the gage line of the spindle as “L” and the sensor length ofthe touch probe 60 as “I”, the displacement angle ε about the A-axis canbe determined from the sum “L+I” and the distance (error) “e” betweenthe points P,P′.

In the subsequent step S5, comparison is made between the displacementangle ε about the A-axis and a predetermined tolerance α to determinewhether the displacement angle ε is within the tolerable range. If thedisplacement angle ε is within the tolerable range, then the pivot angle“θ1−ε” about the A-axis is stored in the storage unit 44 (step S6), andthe process is returned from step 7 to step 1 to proceed to measurementof the next inclined hole.

If the displacement angle ε is not within the tolerable range, theprocess proceeds to step S8. In step S8, the target value of the pivotangle about the A-axis, which has been set at θ1,is corrected to θ1−εusing the displacement angle. The process is then returned to step S2,and the measurement procedures of steps S2 to S5 are repeated with thepivot angle “θ1−ε” about the A-axis, and a determination is made as towhether the re-determined displacement angle ε about the A-axis iswithin the tolerable range (not exceeding the tolerance α). If thedisplacement angle E is not within the tolerable range, the measurementprocedures are again repeated.

When the displacement angle ε has come to fall within the tolerablerange after the repetition of the measurement, the corrected pivot angleθ1 about the A-axis is stored in the storage unit 44 in step S6.

The same measurement and calibration process is executed for the nextinclined hole A2. Thus, the spindle head 10 is pivoted about the A-axisby the same angle as the inclination angle θ2 of the inclined hole A2,and the center position of the calibration gage 62 is measured. Themeasurement is repeated until the displacement angle ε falls within thetolerable range, and the corrected pivot angle θ2 about the A-axis isstored in the storage unit 44. The same process is executed also for allthe other inclined holes.

FIG. 9 is a flowchart showing the flow of a process of a measurementprogram that performs calibration to determine a correction amount ofthe angle of rotation about the C-axis when the table 12 is rotated.

FIGS. 10A through 10C are diagrams illustrating a change in the positionof the table 10 upon execution of the measurement program.

FIG. 10A shows the initial position of the table 12 (angle of rotationabout C-axis=0°) . On the table 12 are mounted the calibration gages 64a, 64 b at points P, Q which are 180-degree symmetrical with respect tothe rotation center O of the table 12.

The first steps S10 to S13 are to determine the position of the rotationcenter O of the table 12. In particular, in steps S10 and S11, theposition of the point P in the initial position of the table 12 (angleof rotation about C-axis=0°) is measured. In the measurement, a macroprogram for automatic centering (two-diametrical direction approach) tomeasure the center of the bore of each of the calibration gage 64 a atthe point P and the calibration gage 64 b at the point Q with the touchprobe 60, is executed to determine the coordinates of the point P whenthe angle of rotation about C-axis is 0 degree.

In steps S12 and S13, the table 12 is rotated 180 degrees, and thecoordinates of the point P′ are determined (FIG. 10B). The position ofthe rotation center O of the table 12 coincides with the midpoint of theline connecting the points P and P′ (step S14).

Next, the angle of rotation of the table 12 about the C-axis is set tocoincide with the direction φ of an inclined hole of the mold (stepS15). The rotation angle is first set at the direction φ1 of the firstinclined hole A1.

Thereafter, the table 12 is positioned at the initial position shown inFIG. 10A (step S16).

Next, the table 12 is rotated until the angle of rotation about theC-axis reaches the direction φ1 of the first inclined hole A1 (stepS17). In step S18, the macro program for automatic centering is executedto measure the positions of the calibration gages 64 a, 64 b lying atthe points P, Q on the table 12 (FIG. 10C).

Once the measured coordinates of the calibration gages 64 a, 64 b areknown, the actual rotation angle φ′1 can be calculated.

In the subsequent step S19, a displacement angle δ about the C-axis isdetermined from the difference between the theoretical rotation angle φ1and the actual rotation angle φ′1. Comparison is then made between thedisplacement angle δ and a tolerance β to determine whether thedisplacement angle δ is within the tolerable range, i.e. not exceedingthe tolerance β (step S20). If the displacement angle δ is within thetolerable range, then the rotation angle φ1 about the C-axis is storedin the storage unit 44 (step S21), and the process is returned from step22 to step 15 to proceed to measurement of the next inclined hole,whereas if the displacement angle δ is not within the tolerable range,the process proceeds to step S23. In step S23, the target value of therotation angle about the C-axis, which has been set at φ1,is correctedto φ1−δ using the displacement angle. The process is then returned tostep S16, and after returning the table 12 to the initial position, themeasurement procedures of steps S16 to S23 are repeated with therotation angle “φ1−δ” about the C-axis. The measurement and calibrationprocess is repeated until the re-measured displacement angle δ fallswithin the tolerable range.

When the displacement angle δ has come to fall within the tolerablerange, the corrected rotation angle φ1 about the C-axis is stored in thestorage unit 44 in step S21.

The same measurement and calibration process is executed for the nextinclined hole A2. Thus, the table 12 is rotated about the C-axis to meetthe direction φ2 of the inclined hole A2, and the positions of thecalibration gages 64 a, 64 b are measured, and the corrected rotationangle φ2 about the C-axis is stored in the storage unit 44 when thedisplacement angle δ has come to fall within the tolerable range. Thesame process is executed also for all the other inclined holes.

Such pivot angle about the A-axis and rotation angle about the C-axis asto make the respective displacement angles within tolerable ranges aremeasured and stored in the storage section 44 in the above-describedmanner for all the inclined holes A1 to An to be machined in the mold 20before initiating machining of the holes.

A description will now be made of oblique machining as carried out inthe CNC apparatus 40 by execution of a machining program, taking as anexample the case of drilling the pin holes A1 to An for insertion ofextrusion pins in the mold 20.

As described above, by the execution of the measurement programs, it isalready known before machining by what pivot angle about the A-axis thespindle head 10 must be pivoted to bring the spindle head 10 into atilted position precisely meeting the inclination angle θ of eachinclined hole and what angle of rotation about the C-axis the table 12must be rotated to make the spindle head 10 precisely meet the directionφ of each hole.

Upon actual machining, therefore, the CNC apparatus 40 reads out data onthe pivot angle θ about the A-axis and data on the rotation angle φabout the C-axis for all the inclined holes A1 to An, as needed, andwhen executing drilling of each hole, instructs the A-axis control unit48 and the C-axis control unit 49 on the corrected pivot angle aboutA-axis and the corrected rotation angle about C-axis instead of theangle about A-axis and the angle about C-axis instructed in themeasurement program. This enables precision oblique drilling of theholes, with deflection of the spindle head 10, the columns 2 and thecross rail 6 due to gravity and a mechanical error in the table 12 beingcompensated.

The position ensuring system of the present invention enables obliquemachining which is entirely different from the conventional obliquemachining method in which a skilled worker manually performs drilling ofinclined holes after manually performing setup for each hole to meet theinclination angle and the direction of the hole. Thus, the presentsystem makes it possible to carry out literally NC-controlled,high-precision automated machining of all the inclined holes to bemachined in a workpiece merely by executing a measurement program and amachining program which are prepared based on CAD data. Efficientoblique drilling, fully making use of the high speed of a five-axismachine tool, thus becomes possible. A remarkable increase in theefficiency of machining of a workpiece can be realized especially whenthe workpiece is a large-sized mold.

As regards machining of inclined holes, such as pin holes as shown inFIG. 3, various machining methods may be employed. Examples include, intime-series order, spot facing with an end mill, machining of a guidehole with a drill, boring with a gun drill, screw cutting with a tap,etc. The position ensuring system of the present invention is, ofcourse, applicable to any of these processing methods.

1. A position ensuring system for correcting an error in a five-axismachine tool having, in addition to X-axis, Y-axis and Z-axis, an A-axisfor pivoting of a spindle head and a C-axis for rotation of a table,thereby ensuring the position of the spindle head in oblique machiningof inclined holes in a workpiece, said system comprising: an A-axiscalibration system which measures a displacement angle, which is anerror between a target value and a measured value of the pivot angle ofthe spindle head about the A-axis as the spindle head is pivoted to meetthe inclination angle of an inclined hole to be machined in a workpiece,and corrects the pivot angle about the A-axis in such a manner that thedisplacement angle as measured with the corrected pivot angle as atarget value fall within a tolerable range, before initiating machiningof the workpiece, the measurement of displacement angle being carriedout for all the inclined holes to be machined by execution of ameasurement program prepared based on data on the shapes, including theinclination angles and the directions, of the inclined holes to bemachined; a corrected data storage device which stores the correctedpivot angle about the A-axis for each of the inclined holes to bemachined; and an A-axis control system which reads out the correctedpivot angle about the A-axis for each inclined hole upon executing amachining program for the workpiece by means of an NC apparatus, anduses the corrected pivot angle about the A-axis to pivot the spindlehead when executing oblique machining of the inclined hole; wherein theA-axis calibration system comprises: a spherical calibration gage thecoordinates of the center of which are known; a probe mounted to thefront end of the spindle head and having a terminal for contact with thespherical surface of the calibration gage; a position detector whichdetects a contact point between the terminal of the probe and thespherical surface of the calibration gage as the terminal is broughtinto contact with the spherical surface from an arbitrary direction; andan arithmetic unit which calculates the coordinates of a measured centerof the spherical calibration gage from the positions of a plurality ofcontact points between the terminal of the probe and the sphericalsurface of the calibration gage as the terminal is brought into contactwith the spherical surface from a plurality of directions, the positionsof the contact points being measured in accordance with predeterminedmeasurement procedures while keeping the spindle head in a tiltedposition at such a tilt angle about the A-axis that meets theinclination angle of each of the inclined holes to be machined, andcalculates the displacement angle about the A-axis based on thecalculated coordinates of the measured center of the sphericalcalibration gage and on said known coordinates of the center of thespherical calibration gage.
 2. The position ensuring system according toclaim 1, wherein the five-axis machine tool is a bridge-type machinetool having an X-axis for moving the table backward and forward, aZ-axis for vertically moving a cross rail which is supported by a pairof columns and vertically moves on the columns, and a Y-axis forlaterally moving the spindle head along the cross rail.
 3. The positionensuring system according to claim 1, wherein the workpiece is a mold tobe machined with a number of oblique pin holes for insertion ofextrusion pins.
 4. The position ensuring system according to claim 2,wherein the workpiece is a mold to be machined with a number of obliquepin holes for insertion of extrusion pins.